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Restoration of the Hex river to equilibrium

morphological conditions

G.R. Basson

Department of Civil Engineering, University of Pretoria, Pretoria,

0002, South Africa

E-mail: bass-gr@fanella. ee. up. ac. za

Abstract

The Hex river near the town of Worcester, South Africa, has a braided character with widefloodplains, a steep bed slope and high bed load. In June 1996, one of the highest floods onrecord caused severe head cutting, scouring the river bed by two metres which caused instabilityproblems at two bridges. A comprehensive hydraulic study has been undertaken to identify theproblem causes and impacts, and to establish a long-term solution for river equilibrium andbridge safety. Man's involvement in altering the river include : extensive mining of bouldersfrom the main river channel with the aim of reducing flood levels which was found to be themain reason for head cutting, construction of bridges with fixed bed levels, river channelalteration, the construction of groynes on the floodplain, and closing of a major part of thebraided system for agricultural development.

The current braided river system has been modelled with a one dimensionalmathematical model, which could accurately simulate the observed bed degradation due to themining. Restoration of the scoured river bed has been achieved by constructing two weirs on theriver in order to raise the bed level to its natural long-term equilibrium state. The bridges havebeen modified with fixed concrete beds at the elevation of the simulated equilibrium bed profile,with the addition of energy dissipation structures to prevent local scour.

1 Introduction

The Hex river, South Africa, originates in the Hex river mountains and flows

through the Hex river valley, renowned for its table grapes. As the river leaves

Transactions on Ecology and the Environment vol 16, © 1997 WIT Press, www.witpress.com, ISSN 1743-3541

224 Ecosystems and Sustainable Development

the Valley it passes to the east of the town of Worcester before reaching the

Breede river. It is on this lower part of the river which has an extensively braided

character, with floodplains as wide as 500 m, on which this paper focuses.

Originally the river was called the "harum-scarum" (direct translation) river near

the town of Worcester due to the seemingly unpredictable changes in the river

course from low to high flow conditions. A map dated 1891 indicates that the

original course of the river was approximately 5 km to the east of its current

position. Due to closure of that part of the braided system for farming purposes

early in this century, a new morphological equilibrium was established in the

remaining river. The river is however still attempting to break through to its old

course as the cutoff has to be repaired regularly. The river is steep with a high

bed load of boulders 100 to 150 mm in diameter. Typical flood flow velocities

are as high as 3 m/s.

The main route between Worcester and Robertson crosses the Hex river

floodplain just outside Worcester by means of twelve bridges. Canalization and

development on the floodplain have concentrated the flow mainly through three

bridges, located between Worcester and Zwelentemba, on river channels Hexl, 2

and 3.(Figure 1). The main rail link towards the east follows the same route as

the road and at this point runs approximately 20 m away on the northern

(upstream) side. Corresponding rail bridges are provided at all the road bridges.

Site investigations after severe flooding in June 1996 revealed extensive

local erosion and also a deep erosion gulley located approximately 100 m

downstream of the Hexl road bridge. The situation was monitored on a regular

basis and it was soon apparent that the gulley was rapidly progressing upstream.

In view of the uncertainties involved and the importance of safeguarding this

route it was decided to undertake a comprehensive investigation of the Hex river

at this location. The study reach extended for a distance of approximately 2 km

either side of the crossing point. The main objectives of this investigation were

to determine maximum possible erosion depths and also develop effective long-

term remedial measures.

2 Historical impacts on the river

In the current braided river system a number of developments impact on the

river:


Ecosystems and Sustainable Development 225

Flood protection groynes

ZwelentembaBBB

BExcavated river channel

Retrogressive erosion (1996)

#8Model network

Figure 1: Location plan of Hex Rever near Worcester



- residential development on the floodplain has caused high risk of flood

damage, especially at Zwelentemba. The local authority has for some years been

mining boulders from the river in an attempt to lower flood levels.

- vineyards were regularly damaged along the river banks as it changed its main

flow course, which was countered some 30 years ago by the construction of

groynes at regular distances along the river which help to restrict excessive

lateral movement of the river.

- Nine road and rail bridges were constructed in the 1950s straight across the

widest part of the braided river to link the towns of Worcester and Robertson.

Possibly due to local bed scour, the Hex2 road bridge river bed was replaced by

concrete slabs, preventing local scour during floods. This local scour was

associated with non-alignment of the duel bridge piers and bridge constrictions.

- Some 20 years ago it was found that very little flow occurred at the Hexl

bridge during floods and in order to distribute the flood flow mainly between

three bridges, this channel was excavated.

3 Recent flood damage

During 1996 one of the highest flood peaks on record occurred in the Hex river

and resulted in high flood discharge through the river channel Hexl (see Figure

1). Downstream of the Hexl bridges, the river banks were scoured 10 to 20 m

wider, and head cutting lowered the river bed by as much as 2 m, within a

distance of approximately 100 m from the road bridge. Very little flow occurred

at the Hex4 bridges during the flood, and the flow distribution was such that

most flow was through bridges 1, 2 and 3 without overtopping the rail or road.

During the lower flow following the flood, the head cutting of Hexl continued

in a narrow deep channel of 6 m width, which by the end of the rainy season

(October 1996) had cut back to underneath the road and rail bridges across

Hexl. The road bridge pier foundations were scoured by 2 m, with only a narrow

cohesive-boulder mass supporting the bridge piers, which necessitated quick

action to save the bridge. Figure 2 shows a 1987 photograph of the Hexl road

bridge, while Figure 3 shows the impact of head cutting as experienced in 1996

at the same bridge.



Figure 2: View of Hexl Road bridge in 1987, looking upstream

Figure 3: View of Hexl Road bridge in 1996, looking upstream



4 Hydraulic analysis

At first inspection one would expect bed and bank scour during a major flood,

especially of Hexl which has been mechanically opened to the braided river

system. What was however puzzling was the continued retrogressive erosion

during relatively small floods following the major flood, something which has

never occurred before since the construction of the bridges. It was only after

further investigation that the origin of the problem came to light: extensive

boulder mining of the river some 800 m downstream, which has widened the

river by 2 to 3 times its original width, and lowered the river bed by at least 2 m.

This operation commenced 3 years ago with the main aim of lowering flood

levels.

Other impacts on the hydraulics were:

- mining upstream of the bridges, limiting sediment availability and thereby

creating under-saturated sediment transport conditions which result in increased

erosion downstream.

- fixed bridge bed of Hex2 bridges caused reduced flow through this "main"

channel of the braided river system as bed erosion during floods was restricted.

- bridge constrictions, with debris (due to deforestation) accumulated against the

piers, caused supercritical flow conditions downstream of the bridges and

excessive bank scour.

5 Mathematical modelling of the river morphology

The aims with the mathematical modelling of the Hex river system were to

establish the morphological reasons why the river bed changed so dramatically

during recent floods, as well as investigate future remedial actions to be taken.

A one dimensional numerical model suitable to model multiple channels in the

braided river, with the capabilities of modelling non-cohesive and cohesive bed

erosion processes and bed load sediment transport, has been used. Simons and

Richardson remarked on the difficulty of modelling a braided river system:

"The braided stream is difficult to work with in that it is unstable, changes its

alignment rapidly, carry large quantities of sediment, is very wide and shallow



even at flood flow and is, in general, unpredictable". From an inspection of a

series of old aerial photographs, stable main river channels could however be

identified (see Figure 1) which have been used in the multiple channel

modelling.

The model was firstly calibrated against observed flood levels and the

critical condition for bed erosion of the consolidated cohesive bed was

established based on a dominant flood of 460 mfVs (1 in 10 year recurrence

interval) and a survey of 1985 which represents a long-term equilibrium (before

the impact of downstream mining). This calibrated model was then used to

simulate the morphological changes due to the mining activity downstream of

the bridges. A simulated long-term bed profile is shown in Figure 4 which

compares well with the observed bed profile.

Due to the limited field data and the need to give answers as soon as

possible with the risk to road and rail, certain assumptions had to be made with

regards to sediment transport, possible bed armouring and erosion. Although

bed load dominates the sediment transport process in the Hex river, fine

suspended sediment are also present during floods. Old deposits consist of a

conglomerate of fine sediment and boulders at high density, which limits re-

entrainment of the sediment once consolidated.

In the modelling a calibrated critical shear stress for erosion of the

cohesive sediment of 150 N/rn̂ was used, based on the preflood bed conditions,

which is relatively high considering typical values < 10 N/rn̂ for estuaries, or 80

N/m for cohesive consolidated reservoir sediment under flood flushing

conditions. Basson\

6 River restoration to equilibrium morphological conditions

The mathematical model simulation of long-term morphological conditions in

the Hex river indicated the following:

- The river bed at the Hexl bridges would rise naturally, creating a smaller river

gradient upstream and thereby reducing the flood discharge in this channel. In

fact the model indicates that in the long term this channel would want to close

completely, as was the case before it was opened mechanically in the 1970s. In

the short term, however, the Hexl channel and bridges would have to withstand

highly erosive conditions.



ZwelentembaBridge

240.00.

| Worcester - RobertsonRail & Road Bridges (Hex1)

Original bedlevel 1980's

Simulated scouredbed level (Ocf 96)

i Observed scoured bed level i[ due to mining (Oct' 96)

210.00.0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500

CHAINAGE (km)

Figure 4: Hexl longitudinal profile with observed and simulated bed levels



- head cutting should terminate just upstream of the Hexl rail bridge as the

constriction causes some reduction in the sediment transport capacity and

erosive power upstream of the bridge.

Engineering solutions to safeguard the bridges have been based on the

natural river equilibrium:

- stilling basins have been designed directly downstream of each of the road

bridges (Hexl, 2 and 3), which will dissipate energy and prevent local bed and

bank erosion downstream of the bridges.

- the Hexl, 2 and 3 bridge beds have been fixed by mass concrete at the

simulated equilibrium bed levels.

- River banks have been protected by riprap of up to 2 tons at a bank slope of 1:4

(vertical to horizontal)

- The impact of the downstream boulder mining has been mitigated by

constructing two low weirs on Hexl, 200 m and 400 m downstream of the road.

These concrete weirs of 1,2 m drop each will raise water levels during a flood to

what it should have been under natural conditions. The reduced sediment

transport capacity upstream of the weirs will cause deposition of sediment which

will consolidate with time and form a new consolidated bed with morphology

corresponding closely to the natural river equilibrium. Although this project

focused on the road and rail bridges, with localized morphological restoration, an

extended study is underway to address more global changes to the river

equilibrium.

7 Future management of the Hex river

The number of parties involved directly in damage caused by man's changes to

the Hex river, makes an immediate coordinated solution difficult to achieve.

These parties, which include the Provincial Administration Western Cape,

Department of Transport, Worcester Municipality, Spoornet, Department of

Mining, Department of Water Affairs and Forestry, Hex River Irrigation Board,

and Breede River Regional Council, however agree that a coordinated



morphological analysis of the Hex river should be carried out, with the aims to

prevent future structural damage, flooding and river damage.

Future river management should control mining activity (if allowed at

all). In the past, mining was mainly undertaken as it was believed it would lower

flood levels. Ironically, this study has found that the mining caused very little

reduction in flood levels, but had a severe impact on the river network flow

distribution, degradation of the bed and sediment transport balance.

Ultimately one would want to restore the river system to its original river

course as was found on old maps dated 1891. This will, however, be quite

difficult due to the socio-economics of current floodplain development.

8 Conclusions

The study showed that a mathematical morphological model can be used

successfully to model the most complex nature of a braided river system.

Furthermore, the impacts of man's involvement with the river, such as mining,

could be simulated accurately, and mitigation to natural equilibrium conditions

has been possible. Future bridge safety has been addressed by first ensuring the

long-term equilibrium river morphological conditions are met, thereby limiting

the impacts of the river on the bridges and vice versa.

9 Acknowledgements

The author is indebted to the Worcester Municipality, the Provincial

Administration Western Cape and BKS Consulting Engineers for providing the

information required to produce this paper.

References

1. Basson, G.R. Hydraulics of reservoir sedimentation, PhD dissertation,

University of Stellenbosch, 1996.

2. Simons, D.B. & Sentiirk, Sediment Transport Technology, Water and

Sediment Dynamics, Water Resources Publications, Colorado, USA,

1992.


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224 Ecosystems and Sustainable Development - WIT Press · PDF filemathematical model, ... 224 Ecosystems and Sustainable Development ... In view of the uncertainties involved and the